US 6934265 B2
Interference reduction with a current-mode transversal filter having taps including binary current sources is provided. Each binary current source provides an output current having either of two distinct values, depending on a binary input. The product of a tap coefficient and an interference data signal value is obtained by independently generating the contribution from each interference data bit using a binary current source and providing these contributions to a current summing junction. Binary current sources can be implemented in analog, digital, or mixed-mode circuitry. Echo, near end cross talk, and far end cross talk are examples of interference that can be reduced in this manner. The use of binary current sources provides significant flexibility, especially in connection with multilevel modulation schemes such as pulse amplitude modulation (PAM).
1. A receiver for receiving an input signal including a data signal and an interference signal derived from an interference data stream, the receiver comprising:
a voltage to current converter for converting said input signal to an input current signal;
a data to current converter comprising a plurality of binary current sources, wherein each of said sources receives a binary data bit input from said interference data stream, wherein each of said sources provides a corresponding output source current having either of two values responsive to said binary data bit input, and wherein said output source currents are summed to provide a correction current signal;
a current summing node receiving said input current signal and said correction current signal and providing a processed current signal;
whereby interference from said interference signal is reduced in said processed current signal.
2. The receiver of
3. The receiver of
4. The receiver of
5. The receiver of
6. The receiver of
7. The receiver of
8. The receiver of
a transmitter providing a transmitted signal to said channel, and
an echo-canceling hybrid connected to said transmitter and to said channel.
9. The receiver of
10. The receiver of
11. The receiver of
12. The receiver of
13. The receiver of
14. The receiver of
15. A method for receiving an input signal including a data signal and an interference signal derived from an interference data stream, the method comprising:
a) converting said input signal to an input current signal;
b) providing a data to current converter comprising a plurality of binary current sources, wherein each of said sources receives a binary data bit input from said interference data stream, wherein each of said sources provides a corresponding output source current having either of two values responsive to said binary data bit input;
c) summing said output source currents to provide a correction current signal; and
d) summing said input current signal and said correction current signal to provide a processed current signal;
whereby interference from said interference signal is reduced in said processed current signal.
This application is related to and claims priority from U.S. provisional application No. 60/506,215, filed Sep. 25, 2003, entitled “Echo Cancellation Method and Implementation for High-speed Full Duplex Communication Systems” and hereby incorporated by reference.
This invention relates to cancellation of an interfering signal in communication systems.
Communication can be regarded as a problem of recovering a desired signal from an input signal that includes undesired signals in addition to the desired signal. Undesired signals include random noise, as well as interference signals which are not random. In many cases, interference signals are generated within a communication system by hardware limitations. For example, when a 2-wire channel is used for full-duplex communication, the signal transmitted to the channel generates an interference signal (often referred to as “echo”) which interferes with reception of signals from the channel. The effect of interference signals which are generated within a communication system (e.g., echo) on reception can generally be reduced because such interference is derived from signals which are known within the communication system.
More specifically, one can regard a communication system as providing a parasitic system having an interference data stream as its input and the undesired interference signal as its output. For example, an imperfect hybrid in a 2-wire communication system provides such a parasitic system, where the transmitted data stream on the 2-wire channel is the interference data stream. Since communication systems are usually linear, the effect of the interference signal provided by the parasitic system can be reduced by deriving an appropriate correction signal from the interference data stream and adding the resulting correction signal to the received signal.
The correction signal is usually derived from the interference data stream by passing it through a linear filter. In cases where the parasitic system leading to the interference is time-invariant, the correction filter can also be time-invariant. Otherwise, the correction filter is usually time-dependent and is placed within a control loop for varying the filter parameters to minimize the contribution of the interference signal to the corrected received signal. Such filters are often referred to as adaptive filters.
Correction filters as described above are frequently implemented by passing the input signal though a tapped delay line. Each tap of the delay line corresponds to a different time delay applied to the input signal. For example, tap 1 could correspond to a delay of T0, tap 2 to a delay of 2T0, etc. In this architecture, the overall filter output is obtained by multiplying each tap output by a corresponding tap weight, and adding the resulting terms. Such filters are also known as transversal filters.
Transversal filters as described above have been known for some time, and thus numerous implementations are known in the art. For example, an extensive body of work relates to reducing the computation time required for digital transversal filters, which is mainly determined by the required multiplications. Such work includes the use of filters having tap weights that are exact powers of 2, so that multiplication can be performed by simple bit shift operations. In addition to such mathematical investigations, various physical implementations of transversal filters have been demonstrated. For example, in the common case where a transversal filter is implemented electrically, mathematical signals can be related to circuit voltages or to circuit currents.
Although voltage-mode transversal filters are more common than current-mode transversal filters, current mode filters can be advantageous in certain cases. U.S. Pat. No. 6,469,988 considers an example of a current-mode transversal filter used for echo cancellation in a communication system having binary modulation. However, many common communication systems employ non-binary modulation, and U.S. Pat. No. 6,469,988 does not consider such cases.
Another example of a current-mode transversal filter for echo cancellation is given in Lee et al., “A 125 MHz Mixed Signal Echo Canceller for Gigabit Ethernet on Copper Wire”, IEEE Journal of Solid State Circuits 36(3), pp 366-373, 2001. In this example, 5 level pulse amplitude modulation (PAM) is employed, and a single digital to analog converter (DAC) is used to provide multiplication at each tap. However, applicability to other modulation formats is not considered by Lee et al. Furthermore, in some cases, it is not practical to perform tap multiplication with a single DAC.
Accordingly, it is an object of the invention to provide current mode circuitry for interference reduction that is applicable to a variety of non-binary modulation formats. Another object of the invention is to provide current mode circuitry for interference reduction that can be used with various multiplier approaches.
The present invention provides interference reduction with a current-mode transversal filter having taps including binary current sources. Each binary current source provides an output current having either of two distinct values, depending on a binary input. The product of a tap coefficient and an interference data signal value is obtained by independently generating the contribution from each interference data bit using a binary current source and providing these contributions to a current summing junction. Binary current sources can be implemented in analog, digital, or mixed-mode circuitry. Echo, near end cross talk, and far end cross talk are examples of interference that can be reduced in this manner. The use of binary current sources provides significant flexibility, especially in connection with multilevel modulation schemes such as pulse amplitude modulation (PAM).
In general, the transmitted signal interferes with the received input signal. Such interference is typically additive, and can be due to imperfections in hybrid 106 (e.g., direct transmission from transmitter 104 to summing junction 112) and/or from reflections within channel 108. Thus hybrid 106 provides an input signal z(t)=r(t)+fint(t) to summing junction 112, where r(t) is the received data signal and fint(t) is the interference signal derived from transmitted data 102
In order to reduce the effect of such interference, transmitted data 102 is also provided to a correction filter 110. The output of correction filter 110, g(t), is provided to summing junction 112 with a negative sign, as indicated on FIG. 1. Summing junction 112 provides a processed signal 114 equal to r(t)+fint(t)−g(t). In order to minimize the effect of transmitted signal interference in processed signal 114, the correction filter output g(t) should be a good approximation to the interference signal fint(t). Methods for designing correction filter 110 to provide such an approximation are known in the art. In many cases, the relation between transmitted data 102 and the interference signal fint(t) varies with time (e.g., reflections in channel 108 can be time-varying). In order to accommodate such time variation, the relation between transmitted data 102 and g(t) also needs to be time-varying, and accordingly correction filter 110 is an adaptive filter. Adaptive filters and methods for their use and control are also known in the art. Correction filter 110 is frequently implemented as a transversal filter, since such filters are relatively simple to implement, especially adaptively.
Many physical implementations of the transversal filter of
In the example of
The tap of
Multiplier 306 has C1n and the data LSB as inputs.
Multiplier 308 has C1n and the data MSB as inputs.
Multiplier 310 has C1p and the data LSB as inputs.
Multiplier 312 has C1p and the data MSB as inputs.
Multipliers 306 and 310 have the same proportionality constant K between output and product of inputs. Multipliers 308 and 312 have the same proportionality constant 2K between output and product of inputs. This arrangement of multiplier proportionality constants ensures that the MSB data bit has twice the effect on the output as the LSB data bit, consistent with the 4-level PAM scheme discussed above.
Multipliers 306, 308, 310, and 312 all have differential outputs connected to a differential current summing node formed by nodes 302 and 304. Multipliers 310 and 312 are connected to nodes 302 and 304 with opposite polarity compared to multipliers 306 and 308. This difference in polarity ensures that the output current on nodes 302 and 304 depends on the difference C1p−C1n, as required. Thus the differential output current on nodes 302 and 304 is proportional to the product of C1 and the data signal 2*MSB+LSB. Thus the tap of
Multipliers 306, 308, 310 and 312 can be regarded as binary current sources (BCS). More specifically, such a binary current source provides either of two currents to its output responsive to a binary input. Typically, several binary current sources are required for each tap, as shown on
The invention is also applicable to various cases not explicitly discussed in the above embodiments. For example, echo cancellation relies on knowledge of the transmitted signal to approximately remove its associated interference from the received signal. Interference from any other known signal can also be approximately removed from the received signal in much the same way. To outline some possibilities, it is helpful to consider a communication system having channels A and B connecting a near end transceiver to a far end transceiver.
Interference in near end reception from channel A due to near end transmission to channel A is an example of echo, as discussed above. Interference in near end reception from channel A due to near end transmission to channel B is an example of near end cross talk (NEXT). Interference in near end reception from channel A due to far end transmission to channel B is an example of far end cross talk (FEXT). Thus NEXT and FEXT are two more examples, in addition to echo, of the types of interference that can be reduced according to the invention. For FEXT, the received signal in channel B can be regarded as equivalent to the far end interfering signal, thus making the interference signal known at the near end.
Although the above examples all show binary current sources having output currents which differ by a factor of two, such a relation between source outputs is not required. Generally, binary current sources outputs can be selected to match the data bits to the modulation scheme being used in practicing the invention. The invention is broadly applicable to various multi-level modulation schemes, such as PAM and trellis coding.